REV3L is the DNA polymerase subunit of pol 6, an enzyme involved in tolerance of genomic damage. REV3L is essential for embryonic development, for reasons that remain to be clarified. We find that Rev3L-defective mammalian cells have high rates of spontaneous chromosome instability, a feature that is also characteristic of many cancers. The PI's laboratory has developed a conditional pol zeta-null mouse, and it appears that a subset of tumors in such mice are preferentially Rev3L-null. Pol 6 may thus be an example of a specialized DNA polymerase that acts as a tumor suppressor gene. The proposed experiments will use a new combination of cellular, genetic, and mouse model approaches to test the extent to which REV3L protects cells against endogenous DNA-damaging stress and chromosomal instability, and to investigate the process of tumor development in the absence of REV3L. The aims are (1) to test the hypothesis that REV3L enables cells to tolerate DNA damage that can disrupt chromosome integrity and interfere with cell proliferation; (2) to determine which domains of the REV3L protein are necessary for normal cell proliferation and genome stability; (3) to test whether loss of REV3L function in mice promotes tumor development.